Mirror micromechanical structures are known, which are made, at least in part, of semiconductor materials and using the MEMS (microelectromechanical systems) technology.
These micromechanical structures are integrated in portable apparatuses, such as, for example, portable computers, laptops, notebooks (including ultra-thin notebooks), PDAs, tablets, and smartphones, for optical operations, in particular for directing in desired patterns light radiation beams generated by a light source.
Thanks to the reduced dimensions, these structures enable stringent requirements to be met as regards occupation of space, in terms of area and thickness.
For example, mirror micromechanical structures are used in miniaturized projector modules (the so-called “picoprojectors”), which are able to project images at a distance or to generate desired patterns of light.
In combination with an image-capturing module, a projector module of this kind enables, for example, implementation of a three-dimensional (3D) photographic camera or video camera for forming three-dimensional images.
The aforesaid mirror micromechanical structures generally include: a mirror element, obtained from a body of semiconductor material in such a way as to be movable, for example with a tilting or rotation movement, to direct the incident light beam as desired; and a supporting element, which is also obtained starting from a body of semiconductor material, is coupled to the mirror element, and has supporting and handling functions. A cavity is made in the supporting element, underneath, and in a position corresponding to, the mirror element, in such a way as to enable freedom of movement for tilting or rotation thereof.
In particular, applications are known in which the mirror micromechanical structure is required to generate a reflection pattern with an extensive field of view (FOV), i.e., a reflection of the incident light beam over a wide angular range.
For example, FIG. 1a is a schematic illustration of an optical projection system, designated as a whole by 1, in which the mirror micromechanical structure 2 is used for reflecting, with a desired angle, an incident light beam, designated by B, coming from a light source 3, for example, a coherent light source of a laser type.
In particular, the mirror micromechanical structure 2, including the mirror element, here designated by 4, and the supporting element, here designated by 5, in which the cavity 6 is obtained, is mounted in such a way that the mirror element 4 is set, at rest, at a wide inclination angle α with respect to the incident light beam B (the inclination angle α being defined as the angle between the direction of the incident light beam B and the normal to the surface of the mirror element 4). This inclination angle may be comprised between 40° and 50°, for example 45°, and evidently corresponds also to the angle at which the incident light beam is reflected by the mirror element 4.
FIGS. 1b and 1c show a respective operating condition of the optical system 1, in which the mirror element 4 is rotated through a negative rotation angle θ (causing, that is, a reduction in the inclination angle α), and, respectively, a positive rotation angle θ (causing, that is, an increase in the inclination angle α), with respect to the resting condition.
It will be noted that the solution described is affected by an important limitation as regards the field of view (FOV) that can be achieved, which cannot guarantee the desired optical performance, at least in given operating conditions.
As shown schematically in FIG. 2, in fact, for positive inclinations of the mirror element 4, which entail values of the inclination angle α greater than a given threshold, a phenomenon of at least partial shadowing or clipping of the reflected light beam may arise, thereby only a part of the reflected light beam may effectively be transmitted towards the outside of the mirror micromechanical structure 2, for example for generation of a desired scanning pattern on an outer surface.
The specific value of this threshold depends on the particular assembly of the mirror micromechanical structure 2. In any case, there is a mechanical rotation angle θ of the mirror element 4 such that the reflected light beam can be at least partially shadowed.
In the example illustrated, this phenomenon is highlighted for a positive rotation angle θ of 20° with respect to the resting condition.
The phenomenon described entails an undesirable deterioration of the performance of the optical system 1. In particular, the optical system 1 may be unable to achieve the desired performance as regards the field of view FOV.
There is a need in the art to solve, at least in part, this problem afflicting mirror micromechanical structures of a known type.